The process of colour printing by photolithography involves the separation of the colours of the image into a number of components (usually four) to be reproduced by printing inks of corresponding colour (usually yellow magenta, cyan and black).
Each colour separation is converted into the form of a halftone dot pattern by which tone rendition is achieved in lithographic printing. The perceived density of a particular colour on the final print depends on the relative size of the halftone dots in that area. It has recently become the practice to carry out both the colour separation and the generation of halftone dots automatically using a colour separation scanner of the electronic dot generation (EDG) type. The four halftone separation images are processed electronically and separately placed onto black and white silver halide films using a scanned laser device. The printing plates are prepared from these four silver images or their duplicates by contact exposure. A further development in this area is the increasing use of electronic pagination systems which can manipulate the digitally stored image data for the purpose of page composition.
A very desirable adjunct to the electronic scanner and pagination systems is a method of producing a colour proof directly from the electronically stored data without the requirement for intermediate black and white images on silver halide film.
Several methods for the production of colour proof directly from electronically stored images are known. It is possible to represent the image on a colour cathode ray tube which may be photographed using any of the commercially available colour photographic materials. Alternatively, a black and white cathode ray tube may be photographed sequentially through different spectral filters. A more sophisticated device which has become available enables the image to be scanned in continuous tone form onto conventional photographic colour paper using blue, green and red light from argon-ion and helium-neon lasers. An additional method is to use the signals to a colour TV monitor to drive a continuous tone scanning device which uses a white light source through red, green and blue filters, to expose a diffusion transfer material.
There are fundamental limitations to the usefulness of the known direct colour proofing methods. In particular, it is not possible to record the image in the exact form that it will finally appear, that is, as superimposed yellow, magenta, cyan and black images of halftone structure.
In one respect this limitation is imposed by the selection of photographic colour materials which are available. All of the silver halide colour recording materials presently available which work by the subtractive principle produce images which are formed from dyes of only three colours: yellow, magenta and cyan.
It is recognized in the printing industry that a colour proof should be an exact representation of the final printed image produced from four superimposed halftone images in yellow, cyan, magenta and black inks. This is not readily feasible using a colour material which cannot form a black image independent of the other colours. To produce a proof using known colour photographic materials the yellow, magenta and cyan images have to be modified to compensate for the absence of a black layer. The result is, therefore, one stage removed from a genuine proof.
A further drawback of known methods employing conventional colour photographic materials is the limitation that the final images are of the continuous tone type rather than the halftone form of the final printed image. Since one of the principal reasons for making a proof is to check whether the sizes of the yellow, magenta, cyan and black halftone dots are correct to produce the desired hue and tone the proof should be composed of halftone dots rather than continuously varying density calculated to produce the same visual effect. The current use of continuous tone exposures is probably dictated by the resolution of the imaging devices in use, the extra equipment cost for computing equivalent yellow, magenta and cyan halftones to the yellow, magenta, cyan and black halftones, the low to medium contrast of commercially available photographic colour materials which makes then not ideal for halftone exposures, and the limited resolution of conventional chromogenic colour paper.
For these reasons the direct colour proofing methods presently available have not achieved widespread acceptance except as a check on page layout and composition. It is still common practice to produce high quality colour proofs either by actually printing on a special press or by laminating together individual yellow, magenta, cyan and black images formed in various ways by contact exposure through halftone separations on black and white film. These methods are generally time consuming and often require a high level of skill on the part of the operator.
Our copending British Patent Application GB No. 2172118A discloses a radiation-sensitive element suitable for colour proofing comprising a substrate bearing at least four separate imaging media coated thereon, said imaging media including:
(1) an imaging medium capable of forming a yellow image upon imagewise exposure and processing, PA1 (2) an imaging medium capable of forming a magenta image upon imagewise exposure and processing, PA1 (3) an imaging medium capable of forming a cyan image upon imagewise exposure and processing, and PA1 (4) an imaging medium capable of forming a black or a balancing black image upon imagewise exposure and processing, PA1 (1) an imaging capable of forming a yellow image upon imagewise exposure and processing, PA1 (2) an imaging medium capable of forming a magenta image upon imagewise exposure and processing, and PA1 (3) an imaging medium capable of forming a cyan image upon imagewise exposure and processing, PA1 (1) an imaging medium capable of forming a yellow image upon imagewise exposure and processing, PA1 (2) an imaging medium capable of forming a magenta image upon imagewise exposure and processing, PA1 (3) an imaging medium capable of forming a cyan image upon imagewise exposure and processing, and PA1 (4) an imaging medium capable of forming a black image or balancing black image upon imagewise exposure and processing, PA1 (1) an imaging medium capable of forming a yellow image upon imagewise exposure and processing PA1 (2) an imaging medium capable of forming a magenta image upon imagewise exposure and processing, PA1 (3) an imaging medium capable of forming a cyan image upon imagewise exposure and processing, each imaging medium having a maximum spectral sensitivity at a wavelength different from that of the maximum spectral sensitivity of the other imaging media within the range 550 to 900 nm, the sensitivities at the wavelength of maximum spectral sensitivity of the media decreasing in order from the medium of shortest wavelength maximum spectral sensitivity to the medium of longest wavelength maximum spectral sensitivity, the difference in said sensitivities between the media of shortest and longest wavelengths maximum spectral sensitivity being greater than 0.8 log exposure units, preferably greater than 1 log exposure unit, more preferably greater than 1.3 log exposure units,
each imaging medium having a maximum spectral sensitivity at a wavelength different from that of the maximum sensitivity of the other imaging media.
The four layer elements are particularly suitable for the generation of highly accurate half-tone colour proofs. The element is exposed by the four independent sources of different wavelengths and image formation in each layer is attributable only to a single source. Thus each layer may be truly representative of the printing plate used to apply the corresponding ink in the printing process.
The elements are based on an entirely different principle to conventional colour photographic silver halide elements. Conventional elements produce a colour image by combinations of cyan, magenta and yellow dyes and the exposing radiation causes image formation with a dye including the wavelength of the exposing source within its principal absorption band. Thus a black image is formed by a combination of all three dyes generated by exposure of different wavelengths and there is no provision for generating black or balancing black by exposure to a single wavelength. The four layer elements utilize false-colour address in order to separate magenta, cyan, yellow and black. Thus the wavelength of the exposing source used to indicate a particular photosensitive layer is entirely independent of the colour generated in that layer. For example, a magenta separation may be digitised and thereafter cause an infra-red sensitive source to expose an imaging layer sensitive to infra-red. This material, on processing, generates a magenta image.
Hitherto false-colour address has been used only for specialised image recording, e.g. infra-red aerial photography and X-ray photography with the exception of U.S. Pat. No. 561892. That patent discloses full colour photographic images are produced by exposure of a radiation-sensitive element comprising at least three silver halide emulsion layers. At least two of which silver halide emulsion layers are sensitised to infrared radiation. Selectively absorptive filter layers and/or differential sensitivities between emulsion layers are used to prevent exposure of other layers to radiation used to expose a single layer.
The imaging media of the elements are selected such that not only does each medium have a maximum spectral sensitivity at a wavelength which is different from the wavelengths of maximum spectral sensitivity of the other imaging media but each imaging medium has a sensitivity at the wavelengths of maximum spectral sensitivity of the other imaging media which is not significant so that upon image-wise exposure of the element to radiation of a wavelength corresponding to the maximum spectral sensitivity of one of said imaging media of sufficient intensity to cause image formation in that medium image formation will be confined to said one imaging medium. Thus, upon irradiation by four independent sources having wavelength corresponding to the maximum spectral sensitivity of the layers and subsequent processing, the elements of the invention form super-imposed yellow, magenta, cyan and black or balancing black images, each image being attributable to the image-wise exposure of the respective source.
The elements can be utilised as a colour proofing system which can produce four-colour, halftone proofs of high accuracy directly from electronically processed separation image data. The digitally processed images are used to modulate independent sources of actinic radiation, e.g. light emitting diodes (LED), laser diodes or infrared emitting diodes (IRED), which are selected to emit at the wavelength of maximum spectral sensitivity of the medium corresponding to the digitally processed image. The four independent exposures may be conducted simultaneously or sequentially since the spectral sensitivities of the imaging media are selected such that exposure from one source will cause imaging formation in one imaging medium but not significantly affect the other imaging media.
British Patent Application GB No. 2172118A also discloses a process for producing a coloured half-tone image comprising providing a light sensitive element comprising a substrate bearing three separate imaging media coated thereon, said imaging media consisting of:
each imaging medium having a maximum spectral sensitivity at a wavelength different from that of the maximum spectral sensitivity of the other imaging media and a sensitivity at the wavelength of maximum spectral sensitivity of any of said other imaging media which is not significant compared to the maximum sensitivity of said other media, exposing said element to three independently modulated sources each emitting radiation of a wavelength corresponding to the wavelength of maximum sensitivity of a respective imaging medium, said exposure being conducted a raster fashion.
It has been found that with a suitable selection of cyan, magenta and yellow image-forming layers of the type described above, it is possible to utilise a three-layer element to produce coloured half-tone images. In general, such elements will not be acceptable for colour proofing since there will be no separate black or balancing black layers and accordingly there will be no direct match with printing inks. However, the elements may be used to prepare high quality coloured half-tone images. The use of a half-tone imaging processing has several advantages over a conventional continuous tone photographic reproduction, allowing more latitude in processing conditions whilst achieving consistency of reproduction to provide latitude for image formation.
The sensitometric contrast of each of the three imaging layers is preferably sufficiently high that the difference between the exposure required to give a density which is 5% of the maximum density above fog and the exposure required to give a density which is 905 of the maximum density above fog is less than 2.0 preferably less than 1.5 log exposure units. The sensitivities of the layers at the wavelength of maximum sensitivity preferably decrease from the layer of shortest wavelength sensitivity to the layer of longest wavelength sensitivity which sensitivity decreases to a value of less than 10% preferably 5% more preferably less than 2% ie. more than 1.0, 1.2 and 1.7 logE units respectively. Generally, the minimum difference in sensitivity between any two layers is at least 0.2 log E units. The three-layer element may be utilised to generate a coloured half-tone dot image by exposure to three independently modulated sources. The black component of the desired image is obtained by combination of the yellow (Y), magenta (M) and cyan (C) in the same manner as in conventional colour photography.
For the three layer system it is preferred that the radiation sources emit at a wavelength at peak intensity in the range 550nm to 900nm and the wavelength separation between any two of the three different wavelengths is at least 20nm.
One way of obtaining a dense black with a three layer (YMC) material (whilst still matching each of the densities of yellow, magenta and cyan to the printing inks) is to coat each of the YMC imaging layers at a higher density. The dye densities would be chosen such that when all three are combined together the result is a dense black. To produce yellow, for example, the exposure of the magenta and cyan layers would be such so as to produce no magenta or cyan, but partial exposure of the yellow layer would be undertaken so as to produce an amount of yellow dye less than its maximum density but sufficient to exactly match the yellow printing ink. The yellow would be used therefore at two densities rather than one. Similar techniques would be used to produce magenta and cyan, or red, green and blue through appropriate combinations of yellow, cyan and magenta.
A disadvantage of this technique is that the resulting colours would be critically dependent on the precise value of the intermediate exposure and on processing variations. However, this problem may be obviated by utilizing two silver halide emulsions of different sensitivities within each colour forming layer.
It is also possible to utilize a four-layer (yellow, magenta, cyan and black or balancing black) positive acting element sensitised to only three different wavelengths to obtain high quality, half tone, full colour images suitable for use as a colour proofing system.
A radiation-sensitive element suitable for the preparation of half-tone colour proofs by exposure to three sources of different wavelength comprises a substrate bearing at least four positive acting imaging media coated therein, said imaging media including:
each imaging medium (1), (2) and (3) having a maximum spectral sensitivity at a wavelength different from that of the maximum sensitivity of the other imaging media of (1) to (3), the imaging medium (4) having spectral sensitivity at each of the wavelengths of the maximum sensitivity of the other imaging media.
The element has four colour-forming layers. A yellow layer (Y) sensitised to a first wavelength, a magenta layer (M) sensitized to a second wavelength, a cyan layer (C) sensitised to a third wavelength, and a black or balancing black layer (K) which is sensitised to all three wavelengths. All four layers are coated onto a base. The order of the layers may be Y, M, C then K, but other permutations are possible. In all cases, however, it is the black or balancing black layer which must be sensitised to all three wavelengths.
The material must be positive acting. In other words, smaller amount of colour-forming component, eg. dye, are present after development in areas which are exposed to actinic radiation of the relevant wavelength, than is present in those areas not exposed. Also, the imaging process must be halftone. Thus, at any given point on the material, each colour-forming component is present at either maximum density of minimum density. For this reason, at any given point on the material the net colour can be colourless (eg. white on a white reflecting base), yellow, magenta, cyan, blue, green, red or black with no intermediate shades. On a macroscopic scale, intermediate shades and colours are produced by the correct dot size of each of the Y, M, C and K.
There is a significant demand for colour hardcopy from electronically stored image data, output in a form which can be re-scanned. This arises from the demand for "second generation originals"; that is, original photographs which have been electronically retouched or otherwise modified and then copied onto fresh colour film to yield a clean `original` which may be sent to other locations and subsequently scanned on a colour separation scanner. THe Dr. Rudolf Hell Gmbh Colour Proof Recorder CPR403 which outputs continuous tone, colour hardcopy on standard colour paper or transparency is presently in commercial use for the production of such second generation originals.
It has been found that a colour proof recording system such as that disclosed in British Patent Application GB No. 2172118A can be modified to include the possibility of producing continuous tone second generation colour originals.
In this case the image produced should match, as closely as possible, the original which may be a colour transparency such as on Kodak Ektachrome or corresponding materials. Only a three colour material is therefore required (yellow, magenta and cyan). To produce continuous tone images the emission sources, eg. LED arrays may be driven at any power level between the minimum and the maximum which would be used for halftone recording. Therefore in order to maximise reproducibility of tonal values, the exposure range of the individual sensitive layers of the material should match the practical intensity range of the emitting eg. LED sources.
Continuous tone colour images may be obtained by a process which comprises providing a light sensitive photographic element comprising a substrate bearing three separate image media coated thereon, said imaging media comprising:
exposing said element to three independently modulated sources each emitting radiation of a wavelength in the region of the wavelength of maximum sensitivity of a respective imaging medium, the maximum emission intensities of the sources at the wavelength of their maximum output increasing from the source of shortest wavelength to longest wavelength by an amount corresponding to the sensitivity difference of the imaging media, said exposure being conducted in scanning, eg.raster or vector fashion and over a number of discrete exposure, preferably discrete intensity levels.
It has now been found that the above described imaging processes which utilize false colour address to produce the colour images may be modified to produce images in the form of an intermediate transparency and the transparency may be used to image a variety of radiation sensitive photographic elements to produce a colour print or four-color proof on true colour generating material, or panchromatic colour separations or colour separated printing plates. By so doing an information record of the colour separations is made in register during the scanning. This intermediate, which need not appear in true colour, contains individual absorbances which then may be used as exposure masks. The final images may be made on a contact exposing frame thus releasing the scanner from the task of making duplicate final copies.